The present invention relates to the field of combined analog and digital signal processing, and more specifically to an analog-to-digital (A/D) system incorporating a self-calibrating gain-ranging amplifier connected to an A/D converter.
It is well known to convert an analog-type signal to a digital signal for purposes of more reliable or convenient data transmission, storage, or manipulation. A frequent expedient, particularly when the analog signal may vary over a large range of values, is to employ a selectable gain (or "gain-ranging") amplifier for selectively amplifying or attenuating the analog signal just prior to digitizing the signal with the A/D converter. The resulting digital output of the A/D converter may then be divided (or multiplied) by the gain (or attenuation) of the amplifier to make the system output proportional to the system input at all settings of the amplifier. A refinement includes making correction for any offset or gain error introduced by the amplifier. Clearly, the overall accuracy of the system is unfavorably impacted by inaccurate knowledge of the gain and offset.
It is further well known that the gain and offset of even the most stable amplifier varies as a result of component aging and environmental factors. Thus, even highly accurate initial calibration of the gain-ranging amplifier is generally not adequate for high precision applications, and manual calibration is both time consuming and difficult. For such cases, there is a need for a simple and effective automatic (i.e., not manual) calibration system and method to permit measurement of the gain and offset of the gain ranging amplifier as required.
To this end, several systems have been devised. A common approach is to periodically introduce one or more known reference signals into the amplifier and compare the digitized output with a pre-established value. Alternatively, a digital-to-analog (D/A) converter and a feedback loop may be periodically incorporated between the output of the A/D converter and the input of the gain-ranging amplifier to permit measurement or adjustment of the gain and offset experienced by one or more reference signals passing through the system. Examples of such systems may be found in U.S. Pat. Nos. 4,143,361 and 4,342,983.
While such systems do indeed provide apparatus for compensating the gain and offset of the gain-ranging amplifier, it should be noted that such measurements are made using the A/D converter as an element of the measuring system, and are therefore limited by the converter's resolution. For a non-varying calibration signal, the A/D converter will provide an output signal no better than the least significant digit of the converter. As a consequence, the overall system accuracy will generally be considerably worse than that of a system having accurate gain and offset correction. This comes about since errors in the gain correction appear as a multiplicative factor whereas errors in the digitization alone and in the offset appear as additive factors.
Accordingly, it is an object of the present invention to provide a simple apparatus for and method of correcting the gain and offset drift of a gain-ranging amplifier to an accuracy greater than that set by the resolution of the A/D converter.